1. Field of the Invention
The present invention relates to a mobile communication terminal and, more particularly, to a mobile communication terminal equipped with a hearing aid function for a user.
2. Description of Related Art
A mobile communication terminal equipped with hearing aid function has been increasingly demanded. According to FCC (Federal Communications Commission) regulation in the United States, mobile communication terminals equipped with no hearing aid function are not permitted to be sold since the year 2001 in the United States.
A hearing-aid is a device used to help a user hear sounds better. It includes a telecoil (T-coil) which is a coil of wire that is switched in place of a hearing-aid microphone to allow the hearing-aid to pickup the varying magnetic field at an earpiece of a telephone handset. The T-coil is essentially one half of a transformer, the other half being provided by the handset or an induction loop in a listening assistance system.
FIG. 1 is a schematic diagram of a receiver of a typical mobile communication terminal. A receiver 10 includes a permanent magnet 11 generating a magnetic field, a pole piece 12 forming a magnetic circuit together with the permanent magnet 11, a voice coil 13 wound around the permanent magnet 11 and the pole piece 12, a vibrating plate 14 provided over the pole piece 12, a frame 15 and a cover 16 which cover the permanent magnet 11, the pole piece 12, the voice coil 13, and the vibrating plate 14. The receiver 10 is connected to a receiver port of the mobile communication terminal to output an audio signal from the mobile communication terminal.
FIG. 2 is a schematic diagram of a conventional receiver with hearing aid function. The receiver 10 includes a hearing aid coil (T-coil) 18 inside the cover 16. As shown in FIG. 2, a plurality of layers of the hearing aid coil 18 is provided over the voice coil 13 so that hearing capability can be improved due to a leakage magnetic field.
In more detail, a part of the magnetic field generated by the voice coil 13 leaks out of the receiver. The magnetic field generated by the voice coil 13 causes an induced electromotive force to be generated on the hearing aid coil 18. The electromotive force has the same frequency as but different magnitude from an audio signal inputted to the receiver 10.
Accordingly, since both an acoustic signal outputted from the voice coil 13 and an acoustic signal caused by the electromotive force induced on the hearing aid coil 18 are outputted from a receiver with hearing aid function, a user can hear sounds better through the receiver with hearing aid function than through a typical receiver.
The receiver with hearing aid function should have an increased impedance to allow a user to hear sounds better. To increase the impedance, the coil has to be thicker, the number of turns of the coil has to be increased, and the receiver has to be thicker to meet the optimum resonance condition.
However, there is a problem in that such a large-sized receiver is not convenient to use. Further, it is not easy to apply a hearing aid function to an ear mike phone with the receiver that allows a user to hear and input sounds.